With the establishment of sugar beet factories early in the century, the pulp resulting from the extraction of sugar from the beets quickly became an important product for feeding cattle, initially by the same farmers who grew the beets. Thus, a beet grower would often deliver a load of beets to the factory and then wait to load his wagon with pulp for feeding to cattle. Such green or wet pulp was usually stored in large pits or silos at the factory, where it remained until it was needed, but this resulted in considerable losses and other problems. Thus, tests have indicated that as much as 65% of the original weight of beet pulp and 50% of its food value may be lost during a six months storage period. In addition, the decomposition and fermentation products of wet beet pulp stored in open pits are offensively odoriferous. Furthermore, as the beet growers' farms became located further and further away from the beet sugar factories, it was necessary to ship the beets to the factories, sometimes considerable distances, with the result that the growers were no longer located at a convenient distance for transporting pulp back to the farm. Since the beet pulp is a result of the slicing of beets into cossettes and the recovery of the sugar by diffusion into water, the resultant pulp is heavily loaded with water. Of course, a considerable portion of the water can be squeezed from the pulp by pressing, but even then the weight of the pulp is such that shipment for more than short distances is uneconomical. Thus, drying of the pulp permits more economical shipment of the pulp to a point of use, as well as overcoming storage loss and odor problems. Formation of the dried pulp into extruded pellets further reduces shipping and handling costs, since dried pulp pellets in bags requires one third to one fourth the storage volume for bulk pulp.
Pulp from the diffusion operation is transported to the presses by a conveyor or by additional water. In the latter instance, excess water is drained through screens prior to feeding into the press. Pressing of the pulp, which removes considerably more water than drying, is normally accomplished by screws surrounded by screens through which the water flows as the pulp is pressed. Such presses include the single screw press of a vertical type and the horizontal twin screw type. Pulp press water is normally returned to the diffusion equipment, sometimes with sterilization to reduce bacteria. Although the pulp drier normally removes a lesser amount of water than the press, the further reduction in water content of the pulp, as from 75% to 85% water on a weight basis, requires considerable energy. Thus, as indicated in the section entitled Dried Pulp of the book Beet Sugar Technology (2nd Ed. Beet Sugar Development Foundation 1971), of the total fuel required by an entire sugar beet factory, as much as 30% may be used in drying pulp. This fuel is normally natural gas or fuel oil, although some driers utilize coal and additional conversions to the use of coal as a fuel may be expected.
In order to increase the value of the dried pulp as a food supplement, various additions have been made to the pulp, primarily of products originating at the sugar factory. Thus, molasses has been added to the pulp, while concentrated filtrate solids from the Steffens house, as well as liquid protein concentrate, have been substituted for a portion of the molasses solids. The pulp with added molasses is referred to as "molasses dried" beet pulp, while pulp with molasses and Steffens filtrate is referred to as "molasses-CSF" beet pulp. In addition, at certain factories utilizing an ion exchange process, non-sugars removed by the ion exchange operation may be added to increase the mineral value of the pulp. The additives are normally added to the pulp, as by spraying, after pressing and prior to feeding to the drier.
A conventional beet pulp drier includes an induced draft, rotating, horizontal axis drum, as on the order of 7.5 feet to 12 feet in diameter and from 31.5 feet to 66 feet long. Drums of 10.5 feet in diameter and 48 feet long are generally considered to be able to dry the equivalent of 2500 tons per day of beets sliced, with smaller and shorter drums having lesser capacities and larger and longer drums having a greater capacity. The drums are equipped with shelves or flights around the inside periphery from which the pulp falls, as a shelf or flight approaches or reaches the top of the drum. However, these shelves or flights are not designed to move the pulp through the drum, so that the mixture of combustion products moving from the furnace through the drum is relied on to move dried or drying pulp from the entrance to the exit end of the drum. The furnace is stationary, so that a sealing section or sleeve between the furnace and the drum is normally utilized. The furnace is normally equipped with one or more burners, such as adapted to burn natural gas or No. 6 fuel oil, as at a 100,000,000 b.t.u. per hour rate for the 10.5 feet by 48 feet drum referred to above. In general, the pulp is dried to a moisture content of 4% to 12% in the final product, with some operators preferring a range of 6% to 8% and some others a range of 4% to 6%, although the latter would appear to be lower than desirable for making pellets.
From the pulp drier drum, the dried pulp and exhaust gases are normally moved into an upright discharge chamber in which the dried pulp settles to the bottom and the exhaust gases are discharged through the top. The inlet of an induced draft fan is normally connected to the upper outlet of the discharge chamber, while the outlet of the induced draft fan is connected to a pipe which transfers the exhaust gases to the cyclone separator, in which normally smaller particles of dried pulp, which have not settled in the discharge chamber, are removed. The dried pulp is removed, as by screw conveyors, from the bottom of each of the discharge chamber and the cyclone separator, for transfer to storage or otherwise, such as pelletizing equipment. Previously, the exhaust gases discharged from the cyclone separator, normally carrying a small amount of dust or fine particles, has been deemed unobjectionable for discharge into the atmosphere. More recently, environmental requirements have necessitated further treatment of the exhaust gases, as in a dust collection system. Water scrubbers have, so far, been the most reliable for removing dust from the exhaust gases in sufficient amounts to satisfy environmental requirements.
Since the temperature of the combustion gases from the burners of the furnace may be on the order of 3500.degree. F. to 3800.degree. F., or 1920.degree. C. to 2090.degree. C., it is desirable to cool the combustion products, to reduce the temperature of the hot gases entering the drum to a temperature on the order of 900.degree. F. to 1500.degree. F., although higher temperatures have been used, but with a greater possibility of pulp combustion in the drum. The exit temperature of the flue gases is reported to vary from 220.degree. F. to 280.degree. F., or 154.degree. C. to 188.degree. C., depending on the installation, while an increase in over all efficiency has been produced by recirculating to the furnace, for cooling the combustion gases, a portion of the exhaust gases rather than ambient air. When a portion of the exhaust gases are returned to the furnace, a connection for this purpose is normally made at the outlet of the cyclone separator. An added advantage of this recirculation is that the reduction of the oxygen content of the gases passing from the furnace decreases the possibility of degradation of the product by oxidation or the combustion of dried pulp, as well as elimination of fine particulates from the stack gases by burning in the furnace.
With the diffusion system operating at capacity and constant supply of pulp being supplied to the presses, the supply of pressed pulp to the drier drum should be essentially constant. However, there are many circumstances which may affect not only the amount of pulp supplied to the drum, but also the moisture content of that pulp. For instance, a change in the acidity of the pulp has been determined as affecting the moisture in the pulp. As a matter of fact, at some plants, acid has been added to the pulp to decrease the amount of moisture in the pulp resulting from pressing. Thus, sulfuric or muriatic acid has been added to adjust to about 5.5 the pH of the pulp liquid about to be discharged from the diffusion system. In addition, microbiological action has been permitted to take place in the diffuser to acidify the pulp. While the action of the latter may cause a loss of sugar in the diffusion juices, this loss may be less than either the cost of the fuel to evaporate the additional water in the pulp drier, or the cost of the addition of sulfuric or muriatic acid to adjust to the same pH. Other additives for the wet pulp which have been used to improve the pressed pulp moisture content include calcium chloride and alum, while the returned pulp press water has also been sterilized by SO.sub.2 to increase acidity. In addition, the pulp may be softer or mushier at one time than another, which tends to result in a higher moisture content, a condition which is also produced from pulp which is too firm. It has been reported that there is a linear correlation between the diffusion battery temperature and the pressed pulp moisture content. Nevertheless, in general, when conditions are such that lower pulp losses and high purity diffusion juices are obtained, the pulp drier operation should be more satisfactory.
The theory of drying pulp is that drying is an evaporation step in which the latent heat of the evaporation moisture in the pulp is supplied by the sensible heat of the hot gases in the drier drum. Thus, a gradient is established between moisture at the surface and the interior of the pulp particles, particularly at the feed end of the drier, when the pulp is wet. Apparently, with sufficient water present, the rate of diffusion of water from the interior of the pulp particles to the surface is rapid enough to maintain the surface in the wet condition, so that evaporation takes place at a temperature below the wet bulb temperature. As the pulp loses moisture, in moving along the drum, the temperature of the drying gases decreases, while the temperature of the pulp tends to increase. Thus, at discharge, the pulp temperature may approach the temperature of effluent gas, although, many times, the pulp temperature may be as much as 100.degree. F. below the wet bulb temperature, which may be in the range of 180.degree. F. to 200.degree. F. for a direct fired drier. Nevertheless, the temperature of the pulp may be as low as 140.degree. F., even though the entering flue gases may be at 1200.degree. to 1500.degree. F.
The evaporation of water from the pulp particles appears to tend to take place at a generally constant rate as long as the pulp remains below the wet bulb temperature, i.e. sufficiently wet therefor. However, when the surface of the pulp particles become drier with a reduction in water content, the evaporation rate decreases and the temperature of the surface of the pulp particles is increased. Fortuitously, the temperature of the drying gas is decreased much lower at the exit end of the drier, so that the increase in the temperature of the pulp is curtailed.
Another factor affecting the drying of pulp is the proportion of small, thin particles of pulp which tend to lose their moisture much more rapidly than larger particles. Such a particle of pulp is heated readily to the temperature of the incoming gases at the entrance end of the drum, which temperature may exceed 1000.degree. F., and such small, thin particles may tend to burn. A counteracting factor is the tendency of the small, thin particles, when dried, to lose considerable weight and thus to be transported quickly by the drying gases to the discharge end of the drum. It has been generally believed that the amount of drying which will take place is proportional to the amount of fuel used by the furnace. Thus, drum driers have been controlled for many decades through an increase in the amount of fuel, when the discharge temperature of the exhaust gases just beyond the drum decreases. Similarly, it has been the practice to decrease the amount of fuel when the temperature of the exhaust gases increases. In general, it is believed that the greatest fuel economy is realized when the pulp drier drum is operating close to its maximum capacity, the reasoning being that the increase of fuel per unit of air or recirculated gas fed to the drier causes a greater temperature difference, which in turn increases the rate of evaporation. Nevertheless, total losses in the drum tend to increase as the rate of evaporation increases. It has also been indicated by tests that the pressed pulp treated with molasses is more susceptible to loss in the drum, although the pulp with molasses does not appear to greatly alter the rate of drying but may tend to extend the zone of an essentially constant drying rate. Nevertheless, the addition of molasses appears to reduce the actual load on the drum, calculated on the basis of bags of dried pulp produced, since the molasses treated pulp contains a higher proportion of dry substance which more nearly approaches that of the dried pulp.
The desirability of effective control of the pulp drying process has been recognized for years. The earlier method of controlling the pulp drier process involved manual control by the operator, principally by increasing the fuel supplied to the furnace, increasing the air supplied by the forced draft fan for the furnace and increasing suction of the induced draft fan for the pulp drier when the pulp became wetter. This manual control was based primarily upon the "feel" of the dried pulp, i.e. an estimate, based upon the operator's experience, of the percent moisture in the dried pulp. Automatic controls have been utilized to regulate the amount of fuel fed to the furnace, with a corresponding control of the forced draft fan, in accordance with the temperature of the heated gases discharged from the drier, normally measured at the outlet of the induced draft fan, but sometimes measured in the drier discharge chamber. This type of control operates very well when there are only small load fluctuations, but leaves much to be desired when the load fluctuations are greater. Attempts have also been made to develop a device for measuring the water content of the dry pulp, such as based on infra red, microwave and electrical conductivity measurements, but these have not been successful. A computer control based upon a weighing mechanism which continually weighed the entering pulp has also been developed, but the expense of the use of this computerized equipment tends to overshadow the savings which might be made thereby. While it has been recognized that the differential in pressure between the furnace and the discharge of the pulp drier is related to the load on the pulp drier, no use has been made of this factor. Thus, in the section entitled Dried Pulp of Beet Sugar Technology, supra, it is stated: "A temperature controller that would alter its control point in direct relation to change in the load is needed." However, there is no suggestion of how such a controller should be made.
Among the objects of this invention are to provide a method of and apparatus for controlling the operation of a rotating drier supplied with hot gases, which method and apparatus are particularly responsive to variations in load; to provide such a method and apparatus which will produce a more uniform moisture content of the product; to provide such a method and apparatus which will reduce the amount of fuel used and concommitantly reduce the cost thereof; to provide such a method and apparatus which involves equipment, the cost of which may be recovered through fuel savings in a comparatively short time; to provide such a method and apparatus which is not only efficient but also effective at substantially all times; to provide such a method and apparatus which may be used with greater safety; and to provide such a method and apparatus which will have additional advantages.